System for the simultaneous operation of a plurality of terminal equipment at a network terminating unit of a broadband network

ABSTRACT

This system comprises a passive bus system which, for the incoming direction (NT→TE), contains a ring-shaped bus, whereby the device connector lines branched off it, have the same signal transit times, and for the outgoing direction (TE-NT) contains a bus line terminated in a low reflection fashion. For the combination of the bit currents flowing from, or respectively to the terminal equipment, a time division multiplex super frame is transmitted with the standard bit rate of 139,264 Mbit/s, which, in both directions, is composed of two 8704 bit frame containing a complete picture line respectively, wherein B bits are contained for the two alternately transmitting ISDN-narrow band channels (B1) and (B2) and bits for the frame synchronization (Rahm-Sync), or respectively segment synchronization (Vor). Compared tothe fraame in the incoming direction, the frame in outgoing direction is shifted by 80% of the frame period, and contains one prefix (Vor) respectively between the signals of various sources reaching the receiver of the network termination unit (NT) with different phase relation, this prefix comprising a clock and a subsequent synchronization bit pattern. The D-channel-signalizing bit (D) is transmitted at the frame start and is expanded at least 10-fold and pseudoternarily coded for the purpose of clearly indentifying overlapping impulses of different simultaneously active sources.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention is directed to a data transmission system which has animproved data format.

2. Description of the Prior Art

Such a system is known in the form of what is referred to as theISDN-S_(O) or S/T interface for the ISDN base terminal of an integratedservices digital telecommunications network (Fernmeldepraxis, No. 19/20,1985, pp. 759-762).

Data transmission systems which utilize bit rates of less than about 200kbit/s are known in the prior art. The subscriber-line technology in theintegrated services digital network is divided into a first area ofswitching to the subscriber, and into a second area of the installationat the subscriber. A network termination unit NT with a U-interface onthe network switching side and a S/T-interface on the subscriber's side,installed at the subscriber, serves as a tie between the two areas. Aconnection between the network termination unit NT and up to eightterminal devices of the same kind or of a different kind occurs via twopairs of a S-bus. The transmission at the S/T-interface occurs with abit rate of 192 kbit/s.

SUMMARY OF THE INVENTION

The object of the invention is to expand such a system such that it isemployable up to bit rates of 140 Mbit/s upon retention of itsperformance features, particularly of the D-channel access control whichis yet to be set forth.

FIG. 1 illustrates the stated object. The passive bus comprising, forexample, eight terminal equipment receptacle sockets and the networkterminating unit (NT) as interface to the ISDN which is merelyschematically shown in FIG. 1 is to be expanded and re-designed such bythe invention that it can process bit rates of about 140 Mbit/s insteadof bit rates under 200 kbit/s, i.e. can process, a data rate which ishigher by about 700 times. In addition to the D-channel access controland the line lengths for the bus and feeders which are possible in thenarrow-band execution, uniform terminal equipment receptacle sockets arealso to be provided both for the previous ISDN narrow-band services aswell as for the newly added broad-band services. The various, possibleconnections are illustrated in FIG. 1, namely:

Terminal equipment for narrow-band services (TE SB),

Terminal equipment for broadband services (TE BB),

Terminal equipment for narrow-band and broadband services (TE SB/BB)

As well as for terminal equipment of the prior art structure fornarrow-band services (TE SBA) which are to be connected via a specialadaptor.

A number of problems caused by the significantly higher bit rate whichdo not occur in the previous interfaces derive in order to achieve arated condition shown in FIG. 1. The properties of the known S_(O)interface shall be discussed in brief below.

Two independent, transparent B-channels (useful channels) each having arate of 64 kbit/s according to CCITT recommendation I.412 and oneD-channel (signaling channel) having a rate of 16 kbit/s are madeavailable in both directions via the S_(O) interface.

The said channels and further functions are combined into a digitalmultiplex signal.

The multiplex frame is 48 bits long and lasts 250 μs. A plurality ofterminal equipment can simultaneously access the D-channel, whereasevery D-channel can be occupied by only one terminal equipment. Aprocedure was selected for the access onto the D-channel which assuresthat a terminal equipment can always successfully transmit itsinformation (D-channel access control) even given simultaneous access oftwo or more terminal equipment to the D-channel. When no information isto be transmitted in the D-channel, continuous status binary 1 istransmitted (quiescent condition). A D-echo channel having 16 kbit/sfrom the network terminating unit in the direction toward the terminalequipment is used for the D-channel access control. Upon receipt of abit from the terminal equipment on the D-channel, the networkterminating unit should reflect this condition with the next D-echochannel bit to the terminal equipment. Every terminal equipment in theactive condition should observe the D-echo channel. It may only accessthe D-channel when, by counting the successive statuses of binary 1 onthe D-echo channel, it has determined that no other terminal equipmenthas occupied the D-channel. At most, six statuses of binary 1 can followone another when the D-channel is occupied. A greater plurality ofsuccessive status of binary 1 means that the D-channel is free. When twoor more terminal equipment simultaneously transmit information on theD-channel, then the pulses superimpose on the line. As long as theterminal equipment transmit identical pulses, the network terminatingunit receives the information as though they came from only one source.Given divergent, transmitted pulses, a negative pulse arises on the linedue to superimposition, for the matching bits in the multiplex frame seeto it that the bits in the D-channel are transmitted only with signallevel 0 or with negative signal level. The pulse is recognized by thenetwork terminating unit and is reflected on the Deecho channel. Theterminal equipment must observe the received D-echo channel during theinformation transmission in the D-channel and must compare therespectively last bit transmitted to the next available D-echo bit. Whenthese two bits coincide, the terminal equipment is allowed to continuethe transmission. Otherwise, the terminal equipment must immediatelyabort the transmission and must return into its observation status.Given simultaneous access of two or more terminal equipment onto theD-channel, that terminal equipment which transmits the binary 1 whereasanother transmits binary 0 is respectively eliminated. Given theabove-described ISDN pilot project, only a few narrow-band serviceshaving bit rates of 64 kbit/s are transmitted in a time-divisionmultiplex method, so -.that this project is also referred to asnarrow-band ISDN for short. The bit duration in the multiplex frameamounts to 5.2 μs. In this relatively long time, disturbances due tointer-symbol interference (modification of the pulse shape due toprecursors or, respectively, post-pulse oscillations of neighboringpulses) do not come to bear since these disturbances decayed long agofor the decision at the receiver in the bit middle. As a consequence ofthe short length of the bus which is about 150 meters, neither thefrequency-dependent degree of attenuation nor the frequency-dependentenvelope delay time of the transmission medium have a noteworthy effecton the pulse shape since the transit time on the bus is significantlyshorter than the duration of a bit. For the same reason, chronologicalshifts of the bits are uncritical within certain limits for the bitsuperimposition on the D-channel.

When ISDN narrow-band services are then to be transmitted together withbroadband services (for example, picture telephony comprising 135Mbit/s) in a common time-division multiplex frame (broadband ISDNISDN-B), then a passive bus in the same form as in the narrow-band ISDNis no longer possible.

Given a bit duration on the order to 10 ns (corresponding to 140Mbit/s), the following disturbances occur:

(a) Transit time difference: The signal transit time between theterminal equipment at the greatest and that at the shortest distancefrom the network terminating unit can be considerably greater than 1/2bit duration. This difference in transit time takes effect in bothtransmission directions, so that it is twice as great at the networkterminating unit. The multiplex frames transmitted from terminalequipment at different distances from the network terminating unit canarrive at the network terminating unit chronologically shifted by morethan one bit relative to one another. A pulse arriving at the networkterminating unit can then no longer be unambiguously assigned a bitposition in the frame.

(b) Inter-symbol interference (ISI): As a consequence of thefrequency-dependent, linear distortions of the transmission medium,precursors and post-pulse oscillations can arise with respect to thepulse centers of gravity, these then extending into the centers ofgravity of the neighboring pulses and potentially simulating falsesignals there. A complete elimination of the precursors and post-pulseoscillations is only possible for a fixed length of the transmissionmedium. The pulse shaping can be undertaken such that an amplitude-wisepulse recognition is still possible even given divergent lengths.

(c) Phase shift of the pulse centers of gravity due tofrequency-dependent envelope delay time: As a consequence of thefrequency-dependent envelope delay time of the transmission medium, thepulse centers of gravity arriving at the network terminating unit fromtransmitters at different distances have different phase relations. Theclock in the receiver of the network terminating unit may therefore notbe phase-locked but must be able to adjust to changing phase relations(clock phase matching).

(d) Disturbing overlay of pulses of simultaneously active sources: Aplurality of transmitters can be simultaneously active in the D-channel,but the receiver in the network terminating unit cannot simultaneouslyadjust to a plurality of phase relations. Moreover, the voltage-relatedoverlay of already distorted pulses having the bit duration of about 10ns leads to an even greater deformation of the resulting pulse, so thatan error-free detection becomes impossible at the high bit rate.

Apart from the inter-symbol interference for which a separate solutionis proposed, the above-recited disturbance possibilities a, c and d areovercome by the feature combination recited in patent claim 1 and, thus,the object of the invention is achieved. Advantageous developments arerecited in patent claims 2 through 7.

Advantages

The differences in transit time are compensated by the ring-shaped busstructure in one transmission direction. The section synchronizationallows a reliable synchronization of signals successively arriving fromdifferent sources, even signals having different coding. As a result ofa significant increase in the bit duration and of AMI-like coding of theD-channel bit in outgoing direction, an unambiguous recognition ofoverlay signals from simultaneously active sources is possible.

Taken together, all measures allow the retention of the D-channel accesscontrol despite the bit rate increased about 700-fold and allow the useof uniform terminal equipment receptacle sockets for narrow-band andbroadband services in a broadband network (ISDN-B). Transmissionequipment are already available for the transmission bit rate of 139.264Mbit/s standardized by the CCITT.

The invention shall be set forth in greater detail below with referenceto nine figures. Shown are:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram for explaining the invention;

FIG. 2 illustrates the basic structure of a broadband, passive bussystem of the invention;

FIG. 3 illustrates the functioning of the section synchronization withreference to a pulse diagram;

FIG. 4 is a pulse diagram which illustrates the overlay of AMI-codedpulses given simultaneous access of a plurality of transmitters on theD-channel;

FIG. 5 is a block circuit diagram of the receiver (4) of the networkterminating unit (NT) of FIG. 2;

FIG. 6 is the structure of the multiplex frame for both transmissiondirections;

FIG. 7 illustrates the chronological offset of the out-going super-framein comparison to the in-coming super-frame;

FIG. 8a shows the connection of a previous narrow-band ISDN terminalequipment to the broadband bus;

FIG. 8b is a time diagram for the E-bits or, respectively, D-bits on theline sections shown in FIG. 8a.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 2 shows the structure of the broadband, passive bus system as wellas the functions of the network terminating unit (NT). The bus system iscomposed of three lines proceeding in parallel of which the upper twoare interconnected such that at their end that a loop arises. The thirdline is terminated in low-reflection fashion at its end.

The transmitter (1) in the network terminating unit (NT) transmitsmultiplex frames comprising synchronizing word onto the loop to allconnected receivers of the subscriber terminal equipment (TE1 throughTEn) which each also contain a controlled transmitter. The receivers areconnected to the returning line of the loop and receive the signalconducted via the loop to the end of the bus. The transmitters of thesubscriber terminal equipment (TE1 through TEn) transmit their signal tothe network terminating unit (NT) on the direct route via the thirdline. What is therewith achieved in addition to a de-coupling of thetransmission directions is that the sum of the signal transit times fromthe network terminating unit (NT) to the receivers of the subscriberterminal equipment (TE) and from the transmitters of the subscriberterminal equipment (TE) to the network terminating unit (NT) is the samefor all subscriber terminal equipment regardless of the respectivedistance between network terminating unit (NT) and subscriber equipment(TE).

Both the subscriber terminal equipment (TE1 through TEn) as well as thereceiver (4) in the network terminating unit (NT) are synchronized tothe transmitter (1). A separate receiver (2) for the synchronizing wordis connected at the loop end in the network terminating unit (NT). Via acontrol (3), this separate receiver (2) synchronizes the chronologicalexecution in the receiver (4) for the signals of the subscriber terminalequipment (TE).

In its left-hand part, the pulse diagram of FIG. 3 contains a portion ofthe time-division multiplex frame in which the transmitter m is activeand the right-hand part thereof contains a portion in which thetransmitter n is active. A phase skip generally arise at the receiver ofthe network terminating unit at the boundary between two time segmentsin which different sources are active. In order to give the receivertime to set to the new phase relation, a time span (pre-span) whichcontains no useful information is additionally inserted between thesections. The pre-span is composed of two parts. The first part isformed by a clock bit pattern comprising many clock information, i.e.comprising many edge changes of the pulses. As a result thereof, theclock generator for the receiver is set to the new phase relation (clockphase matching).

The length of the bit pattern is determined with the transmission code(number of edge changes per bit) and with the quality of the clockoscillator. It is generally true: The oscillator requires more bits inorder to be set to the new phase relation the higher the quality of theoscillator.

The second part of the pre-span is formed by a synchronizing bit patternin which the beginning of the following time segment for the usefulinformation in the multiplex frame is marked (segment synchronization).It must clearly differ from the clock bit pattern so that the boundarybetween the two bit patterns is immediately recognizable. This boundaryis used as reference point for the segment synchronization. The lengthof the synchronizing bit pattern is based on the transmission codeemployed. Given CMI coding, for example, the clock bit pattern can becomposed of a string of zeros. A single bit having binary 1 (see FIG. 3)then already suffices as the synchronizing bit pattern. The0-1-transition effects the segment synchronization. After the clockphase matching, thus, only the first logical 1 following the zero stringmust be checked in order to identify the beginning of theinformation-containing time segment. It is also shown in FIG. 3 that aprotection zone is established between the two sections of thetime-division multiplex frame in which the transmitters m and n areactive in order to intercept the time offsets of the signals arriving atthe receiver.

In the first line a, the pulse diagram of FIG. 3 shows an example of abit sequence which is to be transmitted. The line b shows a CMI signalof both transmitters at the receiver in the ideal case, withouttime-offset. The line c shows the signal of the transmitter m arrivingat the receiver with a time-offset Δtm. The line d shows the signal ofthe transmitter n arriving at the receiver with a time-offset Δtn. Linee, finally, shows the CMI signal of both transmitters as it arrives atthe receiver in the non-ideal case. It can be seen that the protectionzone can be significantly shortened by unfavorable time-offsets.

In addition to the needed section synchronization, there is also theactual frame synchronization which ensues such that the networkterminating unit transmits a synchronizing word as a frame startcharacter and receives it again via the ring bus. The networkterminating unit is thus set to the respective signal transit time onthe bus.

Also added thereto are the transit time of the terminal equipmentconnecting lines and the response times of the terminal equipment whosesum, however, must always be identical for all terminal equipment. Theratio of information-free times to information carrying times must bekept so small that the increase in the transmission bit rate which iscaused thereby remains acceptable.

In defined bit positions of the multiplex frame, a plurality ofspatially separate transmitters are allowed to be simultaneously active(for example, D-channel bits). The pulses from these transmittersoverlay at the network terminating unit to form an overall pulse. As aresult of the differing phase relation of the in-coming pulses and as aresult of differing, length-dependent amplitude distortions, the overallpulse can no longer always be recognized as error-free. What is achievedby a brief increase in the bit duration from the order of 10 ns to theorder of 100 ns is that the regions of discrimination uncertainty remainlimited to the bit boundaries and a decision is possible in the bitmiddle. This increase in the bit duration means a reduction in themomentary transmission bit rate, i.e. the multiplex frame containsregions of different bit rates. An insertion of an information-free timesegment (pre-span) is not necessary before the bit spread because thetime for the bit decision is fixed with adequate precision andchronological fluctuations are ineffective due to the relatively greatbit width. Various transmission codes that are optimally adapted to therespective regions can also be selected for the regions varying in bitrate. Different transmission codes may thus also be employed within amultiplex frame. FIG. 4 shows an example of the bit spread as proposedhere for the D-channel. The pulse diagram of FIG. 4 shows the overlay ofthe voltage pulses for the D-channel pending at the receiver (4) of FIG.2 in the case where a plurality of transmitters are signalingsimultaneously.

In FIG. 4 line shows the CMI signal of the transmitter m which is activein the time segment preceding the D-bit. As in FIG. 3, the protectionzone then follows. The following section of the time-division multiplexframe which is 36 CMI bits broad is reserved for the D-channel signalingbit.

FIG. 4, line b, shows, 2 bits in modified AMI code are placed in thistime span. The line b shows the AMI signal of a transmitter u whichtransmits in the D-channel, this AMI signal arriving with a leading timeoffset Δtu in comparison to the ideal position (Δtu=0).

FIG. 4, line c shows the AMI signal of a further transmitter v whichlikewise access the D-channel and which arrives with a trailing timeoffset Δtv.

When a terminal equipment has 0 nothing or binary 1 to signal in theD-channel, then the signal level remains 0, as shown in line d. Thetransmission of AMI bits in the D-channel ensues with the dc-freesequence +1-1 or 00.

The sum signal obtained by superimposition of the signals of alltransmitters is shown in line e. Given this signal, the evaluation canoccur, for example, in the middle of the negative bit as shown in linee.

Given a transmission bit rate of, for example, 140 Mbit/s, a momentarytransmission rate of 140 Mbit/s(36/2)26 7.8 Mbit/s results for the timesegment of the bit spread, this corresponding to a basic frequency of3.9 MHz. Given this relatively low frequency, AMI signals can beemployed on coaxial cables about 100 m long as the transmission mediumfor the voltage overlaying.

FIG. 5 shows the function blocks of the receiver (4) shown in FIG. 2 inthe network terminating unit (NT) for signals of different transmittershaving different momentary bit rates. The receiver control (3) shown inFIG. 2 supplies to a frame synchronization for the receiver (4). Itoperates with adequate precision in order to sample the individual timesections in the region of the opened eye. The amplified reception signalproceeds, first, via a ternary decision stage (31) to the register (32)for a D-channel bit where the D-channel bit is strobed at the correcttime specified by the control unit. In addition to receiving the signalshaving low momentary bit rate, the receiver must also receive thebinarily coded signals having a high bit rate from various transmitters.To that end, the receiver must re-adjust to the respective phaserelation at the beginning of each and every time segment. For thispurpose, the reception signal is supplied to a lockable reception clockgenerator (33) which adapts to the respective phase relation and is alsosupplied to a binary decision stage (34) (threshold detector). Thebinary decision stage outputs a binary signal that is conducted to avariable buffer memory (35) and to a synchronizing pattern recognitionunit (36) assures section synchronization.

At a time when, due to the pre-span, the clock must already have lockedinto the correct phase relation, the receiver control unit activates thesynchronous pattern recognition circuit (36). This searches in thereception data stream for the bit pattern for the sectionsynchronization. As soon as it has found it, it starts the followingcounter (37) and deactivates itself. The counter (37) counts off m bitsin the discrete-value reception signal, where m is the plurality ofuseful bits in the respective time segment of the multiplex frame.

The received bits are introduced into a variable-size buffer memory (35)in the counting clock. The buffer memory (35) is read out with a clockgenerated by the control unit. The output clock is thus free of thephase variations of the reception clock.

In the specific instance of the broadband interface (SBB), the followingbit streams must be combined in time-division multiplex:

16 kbit/s for the D-channel in both directions for the signalling;

16 kbit/s for the D-echo channel from the network terminating unit inthe direction toward the terminal equipment for the D-channel accessmethod;

64 kbit/s for the B1-channel in both directions for the usefulinformation;

64 kbit/s for the B2-channel in both directions for the usefulinformation;

Bit stream for the activation/deactivation of the terminal equipmentfrom the network terminating unit in the direction toward the terminalequipment;

135 Mbit/s for the broadband channel in both directions for the movingpicture transmission (picture telephony BiFe);

Bit streams for the frame and line synchronization and potential audioin both directions.

It is already known to employ the standard bit rate of the fourth stageof the PCM hierarchy of 139.264 Mbit/s for this purpose, this beingconstructed on the basis of 2.048 Mbit/s systems. Given a bit raterequirement of about 135 Mbit/s for the moving picture transmission,adequate space still remains for two narrow-band channels and for therequired control and synchronization (NTZ 38(1985), No. 3, p. 143). Theinvention likewise employs this standard bit rate. Since it is amultiple of 64 kbit/s, a stuffing both in the narrow-band channels aswell as, in particular, in the D-signalling channel can be avoided as aresult of the multiplex frame structure set forth below.

The question of the repetition frequency of the multiplex frame nowarises. Insofar as possible, only one D-bit should be contained perframe because of the D-channel access control and, accordingly, only oneD-echo bit should be contained as well. At the same time, it isdesirable to transmit a complete picture line per frame. One arrives ata frame repetition frequency of 16 kHz using these considerations. Theplurality of bits for the two B-channels is thus also determined as fourbits each (=1/2 PCM sample). However, when one wishes to have aword-by-word transmission for the B-channels, then a super-frame havingthe repetition frequency of 8 kHz is formed which contains two framesthat can be distinguished in that the first frame contains thesynchronizing word and the second frame contains the answer back word.The transmission of the B-channels then occurs in alternation, with 8bits (=full PCM sample) in the first frame, for example B1, and likewisewith 8 bits in the equidistant rhythm in the second frame B2.

The time available for the transmission of a frame is 40 ms asdetermined from the frame frequency of 25 Hz. 640 multiplex frames aretransmitted during this time given a multiplex frame repetitionfrequency of 16 kHz. Of these, 625 frames each contain 1 picture line;the remaining 15 frames are free for future employment purposes. Thiscorresponds to a free transmission capacity of 8640 bits ×15 frames×16kHz/640 frames=3.24 Mbit/s. When all lines of a field are transmitted insuccessive time-division multiplex frames and the fields areequidistantly transmitted, then a frame or, respectively, line memorycan be superfluous when using a modern television set which adapts tothe respective line frequency.

The signal coding on the bus is in the CMI code (coded mark inversion:the binary value 1 is alternately represented by a positive and by anegative polarity and the binary value 0, regardless of the precedingbit, and it is represented by a negative polarity in the first half andby a positive polarity in the second half of the bit interval). Only inthe direction from the subscriber terminal equipment to the networkterminating unit is the CMI coding abandoned in the time segment for theD-channel bit for the purpose of pulse overlaying and when a change toAMI code having the greatest possible bit duration is undertaken.

FIG. 6 shows the structure of the time-division multiplex frame for bothtransmission directions. The time segments of each and every channel arethereby equidistantly arranged in the multiplex super-frame. Themultiplex frames or, respectively, super-frames are of identical lengthfor both transmission directions.

The individual time segments of the multiplex frame denote:

Direction coming from network terminating unit → Terminal Equipment:

Frame Synchronizing Word (Frame Sync): The frame synchronizing word isonly transmitted from the network terminating unit to the terminalequipment and identifies the beginning of the first frame in thesuper-frame. It contains 16 bits, this establishing an adequately lowprobability that it will occur again at some other location givensynchronizing in the super-frame. After the terminal equipment haverecognized the synchronizing word, they need not check it again untilafter a full length of the super-frame. Every inherently dc-free bitpattern is allowed for the synchronizing word, for example, the bitsequence 00101110 10011010. The repetition frequency is 8 kHz.

Answerback Word: The second frame in the super-frame from the networkterminating unit in the direction toward the terminal equipment containsan answerback word 16 bits long at the location of the framesynchronizing word and, this answerback word differs from the framesynchronizing word by at least one bit. What frame of the super-frame isthen being transmitted and what B-channel is taking its turn are thusdetermined. The repetition frequency is 8 kHz.

Activation Bit (A) The A-bit tells the terminal equipment whether theconnection between the network terminating unit and the terminalequipment is properly set-up, i.e. the slice 1 of the OSI referencemodel of the ISO is to be considered operationally ready, as in thenarrow-band ISDN (Harold C. Folts, McGraw-Hill's Compilation of DATACOMMUNICATIONS STANDARDS, Edition II, (1982), Part 2, pp. 521-532). TheA-bit is transmitted only from the network terminating unit in thedirection toward the terminal equipment. The repetition frequency is 8kHz.

D-Echo Channel (E): One bit is required for the D-channel accesscontrol, this reflecting the D-bit received in the network terminatingunit. In the active condition, the terminal equipment constantly observethe E-bit and compare it to their transmitted D-bit. That terminalequipment which determines the identity of the two bits is allowed tocontinue transmitting. All remaining terminal equipment have no accessto the D-channel or, respectively, must immediately abort theirtransmission. The D-echo channel has a bit rate of 16 Kbit/s and is usedonly by the network terminating unit in the direction toward theterminal equipment.

D-Channel (D): The D-channel is the shared signalling channel for allservices. It has a bit rate of 16 kbit/s. In the direction toward theterminal equipment, 1 CMI bit for the D-channel is transmitted permultiplex frame. The D-channel bit is followed by 25 free bits (X) forarbitrary utilization which are here inserted in departing direction formatching to the frame.

Arriving Narrow-Band Channels (B1, B2): The useful channels (B1) or,respectively, (B2), follow for the narrow-band services having a bitrate of 64 kbit/s each. 8 bits of the B1 and, respectively, of the B2channel are transmitted in alternation in every frame. The usefulchannel (B1) or, respectively, (B2) is followed by a protection zone bit(G/S) to be explained in detail later.

Synchronizing Bits for Line Position and Frame Start (B-Sync): For thebroadband service (BiFe or TV), a criterion must be cotransmitted forthe picture screen line number or, respectively, for the start of afield or/and frame. 10 bits are reserved for this purpose from thenetwork terminating unit in the direction toward the terminal equipment.That suffices for numbering the 640 single frames.

Useful Channel for the Broadband Service (BiFe/TV): One time segmentcomprising 8640 bits is available in the frame for the broadband service(BiFe or TV in the direction toward the terminal equipment). This bitplurality corresponds to one picture line given the above-presentedmoving picture coding. Of 640 transmitted frames, only 625 frames=625lines are used for the image transmission. In the remaining 15 frames,these time segments are available for other services, as already setforth above. The relationship of 625 frames for picture transmission to15 frames for other services is time-invariable. The bit rate of thisuseful channel amounts to 138.240 Mbit/s.

Departing Direction Terminal Equipment→Network Terminatinq Unit:

D-Channel (D): In the opposite direction, the multiplex frame beginswith the time segment for one D-channel bit (D). It has the duration of36 CMI bits. In it, 2 AMI bits are transmitted with the momentary bitrate of about 7.8 Mbit/s. However, they only carry the information ofone D-bit.

Pre-Span (Vor): Every source that wishes to transmit useful informationmust first send out a pre-span (Vor) of a few CMI bits so that thereceiver in the network terminating unit can set to the new phaserelation of the pulses. The pre-span (Vor) has the bit sequence00000001. The CMI-coded zeros contain much clock information for therecovery of the clock. The change from 0 to 1 informs the receiver ofthe beginning of the time segment to be evaluated (segmentsynchronization). The pre-span (Vor) is only needed when a plurality ofsources at different locations access the passive bus. This is only thecase in the direction toward the network terminating unit. In thedirection toward the terminal equipment, the full frame comes from onesource, namely from the transmitter in the network terminating unit, sothat the pre-span is omitted.

Departing Narrow-Band Channels (B1, B2): The narrow-band channels (B1)or, respectively, (B2) follow, each comprising 8 bits, and these areagain followed by a protection zone bit (G/S).

Broadband Channel: In the departing direction, the time segment for thebroadband channel begins with a pre-span (Vor) that is 8 bits long. Twobits for field and frame identification (B-sync) follow and, as in thearriving direction, the 8640 bits for picture telephony (BiFe) thenfollow. The frame closes with the G/S bit to be set forth below.

DC-Compensation/Protection Zone Bit (G/S): At those locations in themultiplex frame at which one source ceases transmission and anotherbegins to transmit, a bit falsification can occur when the pulse centersof gravity of the signal of the ceasing source arrive tardy and those ofthe signal of the beginning source arrive early at the networkterminating unit (cf. FIG. 3). The information of the last bit of theceasing source can be falsified by pulse overlaying. A protection zonebit (G/S) is therefore inserted at every location in the frame at whichthe jurisdiction of one source having the bit rate 139.264 Mbit/s canend. This applies to the transmission direction from the terminalequipment to the network terminating unit. Since the frame used in theopposite direction should be identically constructed to the greatestpossible degree, corresponding G/S bits are also arranged. Givenemployment of a transmission code which is not dc-free bit-by-bit, thesebits can be used as dc-compensating bits, as is the case given CMIcoding. What is achieved by the G/S bit at the end of the frame for thedirection toward the terminal equipment is that the framesynchronization and answerback words always have the same CMI pulsesequences. As a consequence of the dc-free AMI pulse sequence, noseparate dc-compensating bit is required in the time segment for theD-channel and no protection zone bit is required due to the great bitduration.

The chronological relationship between a arriving and a departingmultiplex frame is shown in FIG. 7 for that case wherein narrow-bandISDN terminal equipment already existing are not to be connected to thebus. The chronological offset of the two frames shown is for has thefollowing reason: The method of D-channel access control presumes thatthe terminal equipment have adequate time for the evaluation of theD-echo bit before transmitting the next bit. Second, the receiver in thenetwork terminating unit must return the received D-bit to the terminalequipment together with the next D-echo bit (E-bit), as marked in FIG. 7by the arrows from D to E. These conditions are met in that themultiplex frame from the terminal equipment to the network terminatingunit is chronologically shifted by about 1/2 a frame length incomparison to the frame from the network terminating unit to theterminal equipment. From the time of the evaluation of the received bitup to the beginning of the time segment for the next E-bit to betransmitted, the network terminating unit has time to decide whatlogical status this E-bit must assume. For example, the evaluation timefor the D-bit in the direction toward the network terminating unit canlie in the middle of an MI pulse. Due to the chronological offset byabout 1/2 a frame between forward and return frame, the decision timesfor the network terminating unit and for the terminal equipment are ofabout equal length.

When S_(O) -interfacable narrow-band ISDN terminal equipment are to beconnectable to the broadband bus of FIG. 8a via adaptors, then theadaptor must also be granted time for the conversion of the signals. Theoverall time available must then be distributed as uniformly as possibleto the network terminating unit (NT), the adaptor and the narrow-bandISDN terminal equipment. What response time the terminal equipmentshould have is already defined in the specification of the S_(O)interface. The time still remaining, as shown in FIG. 8b, is dividedroughly uniformly for the network terminating unit and the adaptor. Atime offset of about 80% of the frame duration thus results, i.e. 50 μs,by which the frame incoming to the network terminating unit is delayedin comparison to the frame out-going from the network terminating unit.

It is assumed for the concept of the multiplex frame that thenarrow-band ISDN terminal equipment already existing should beconnectable to the broadband bus. The time of 50 μs is thereforesuggested for the shift of the multiplex frames. FIG. 8a shows theconnection of a narrow-band ISDN terminal equipment to the passivebroadband bus via an adaptor. The adaptor has the job of converting thebroadband interface (SBB) into the S_(O) interface and vice versa. Thecooperation of the received E-bits with the D-bits to be transmitted isthe determining factor for the chronological shift of the multiplexframes for the two transmission directions. This chronologicalrelationship is shown in FIG. 8b. To this end, the individual linesections are referenced ○1 through ○4 in FIG. 8a.

The time differences Δtij and Δtijk (i,j=1 . . . 4, k=a . . . d) FIG. 8bcontain the time from the beginning of a bit at the respective receiverup to the decision, the processing time and the respective equipment,and the signal transit time up to the receiver of the respectivelyfollowing equipment. Denoting in detail are:

Δt12: Time between arrival of the E-bits at the adaptor and arrival ofthe E-bit at the terminal equipment

Δt23: Time between arrival of the E-bit at the terminal equipment andarrival at the D-bit at the adaptor

Δt34: Time between arrival of the D-bit at the adaptor and arrival ofthe D-bit at the network terminating unit

Δt41: Time between arrival of the D-bit at the network terminating unitand arrival of the next E-bit at the adaptor.

Because each of the 250 μs-long multiplex frames of the S_(O) interfacecontains four E or, respectively, D-bits and these are not equidistant,FIG. 8b shows the time span of a full S_(O) frame which is equivalent to4 frames on the broadband bus. Because of the non-equidistant positionof the E or, respectively, D-bits in the S_(O) frame, four cases must bedistinguished at the transitions from ○1 to ○2 and from ○3 to ○4 , thesebeing indicated with a through d. The respectively shortest timedifference, i.e. Δt12d, Δt34b and Δt41, are the determining factors forthe time offset between the frame ○1 and ○2 or, respectively, betweenthe frame ○3 and ○4 or, respectively, between the frame ○4 and ○1 . Thetime between two successive E-bits in frame ○1 (transmitted from thenetwork terminating unit) is constant and amounts to 62.5 μs. Asproceeds from FIG. 8b, valid is:

    Δt12k+Δt23+Δt34k+Δt41=62.5 μs (for all k).

The time difference Δt23 is rigidly prescribed by the specification ofthe S_(O) interface. It is calculated at:

    Δt23=(250 μs/frame)/(48 bit/frame)×3 bit=15.625 μs.

As may be seen from FIG. 8b, the following equations apply:

    Δt12a+Δt23+Δ34a+Δt41=62.5 μu    (1)

    Δt12a+35 ZE=36 ZE+Δt21d                        (2)

    Δt34a+12 ZE=12 ZE+Δt34b                        (3)

    1 ZE-250 μs/48                                          (4)

In order to grant the participating equipment (network terminating unitand adaptor) processing times of about the same size, the determiningtime differences Δt12d, Δt34b and Δt41 should be of approximately thesame size. Hence, the demand:

    Δt21 d=Δt34 d=Δt41                       (5)

Obtained from equations (1) through (5) is:

    Δt12d=Δt34b=Δt41=12.1527 . . . μs.    (6)

When the signal transit time from the network terminating unit to theadaptor is left out of consideration, then Δt41 is also the time fromthe reception of a D-bit up to the transmission of a E-bit by thenetwork terminating unit. The middle of the E-bit and the middle of thesame D-bit lie at approximately the same location in the respectiveframe. The time offset of the frame should therefore roughly correspondto the time offset of the said bits (Δt41). This value corresponds toabout 1/5 of the frame duration, i.e., the frame transmitted from thenetwork terminating unit is delayed by 20% of the frame duration incomparison to the received frame. Due to the periodicity of the frame,this is equivalent to a delay of the frame transmitted by the terminalequipment or, respectively, adaptor by 80% of the frame duration,corresponding to 50 μs, in comparison to the received frame.

We claim:
 1. System for the simultaneous operation of a plural number ofterminal equipments (TE) by means of bit currents at a networkterminating unit (NT), which has a receiver (4) which receives signalsfrom a common D-channel of different terminal equipment (TE),characterized in that:a. a loop-shaped bus having an outgoing directionportion and an incoming direction portion and extending between saidterminating unit (NT) and said terminal equipments, a single bus linefor connecting said terminal equipments with said network terminatingunit (NT) and terminated in low-reflection fashion at its end, aplurality of equipment connector lines each connected to one of saidterminal equipments (TE) and branched off from said loop-shaped bus lineand said single bus line so as to provide identical signal transittimes; b. said terminal equipment generating signals in respective timeslots and data bit streams flowing from or, respectively, to saidterminal equipment are combined using a time-division multiplexsuper-frame containing two ISDN narrow band channels B1 and B2 which istransmitted with the standard bit rate of the fourth PCM hierarchy levelof 139.264 Mbit/s, said super frame being composed so that when it istransmitted in both incoming and outgoing directions two frames willeach contain a complete picture line and each frame comprises 8704 bitincluding 8 bits for said two ISDN narrow-band channels B1 and B2 anddata and wherein synchronization bits are contained for the framesynchronization or, respectively, segment synchronization (Vor); c. theframe of data transmitted in said outgoing direction is shifted by 80%of the frame duration to the frame of data transmitted in said incomingdirection and contains a pre-span interval between the signals fromvarious sources which arrive at said receiver of the network terminatingunit (NT) with different phase relations, said pre-span interval beingsynchronization bit pattern; d. for the purpose of unambiguouslyrecognizing overlaying pulses from different sources which are active atthe same time, D-channel signalling bit transmitted at the frame startfrom said terminal equipment (TE) to said network terminating unit (NT)is expanded by at least 10 times and is coded in pseudoternary fashion.2. System according to claim 1 characterized in that a timespan of 36CMI bits is provided at the beginning of each of the two frames of thesuper-frame or the D-channel signalling bit (D) output in said outgoingdirection (TE→NT), said time span being provided such that respectively2 bits which have the sequence +1-1 or 00 are transmitted dc-free forevery expanded signalling bit, whereas all other bits of the frameremain CMI coded.
 3. System according to claims 1 or 2, characterized inthat both the transmitted information of the narrow-band channel as wellas those of the broadband channel in said outgoing direction (toward NT)are introduced by a free-span (Vor) composed of 7 bits for the clockphase matching and of 1 bit for synchronization of data.
 4. Systemaccording to claims 1 or 2, characterized in that the picture line ofthe broadband channel comprising 8640 bits in said outgoing directiontoward the network terminating unit is preceded by 2 bits for videosynchronization (b-Sync).
 5. System according to claim 1, characterizedin that the first frame of the super-frame in arriving direction (NT TE)is preceded by a 16 bit frame synchronization word and the second frameis preceded by a 16 bit answerback word which is respectively followedby an activation/deactivation bit (A), which is followed by a D-channelecho bit (E) and by a D-channel signalling bit (D) which is followed by25 arbitrarily employable CMI bits (X) for the compensation of thefree-span (Vbr) for the opposite direction.
 6. System according to claim5, characterized in that the picture lines in said incoming directionare preceded by 10 bits for picture synchronization (B-sync)
 7. Systemaccording to claim 1, characterized in that the individual signalsections of the frames in said outgoing and said incoming directions areeach terminated with a protection zone bit (G/S) which can also beemployed for dc compensation.